The present invention relates to a method for delivery of concrete to a job site, and more particularly to a method by which the dry components of a concrete mixture are prepared in pre-weighed packages at a cement plant, delivered to the job site, and thereafter the dry components are blended together, mixed with water, and used as needed or desired. The logistics of providing concrete for a construction project can be quite complicated. Concrete is a mixture of a “paste” and aggregate, where the aggregate is typically a blend of coarse aggregate (gravel) and fine aggregate (sand). The paste, composed of portland cement and water, coats the surface of the fine and coarse aggregates. The paste hardens and gains strength to form concrete, a rock-like mass. Concrete therefore has the trait of being plastic and malleable when newly mixed, but strong and durable when hardened. Other additives or “admixes” may be added to provide various properties to the concrete, including water reducer, accelerant, retardant, foaming agents, and other density control additives.
Soon after the aggregate, water, and the cement are combined together as a slurry, the mixture starts to harden. During the chemical reaction of the cement with the water (i.e., hydration), a node forms on the surface of each cement particle. The node grows and expands until it links up with nodes from other cement particles or adheres to adjacent aggregates. This process results in the progressive stiffening and hardening of the slurry and the gradual development of strength in the slurry. Therefore, once the cement is placed into contact with water through the mixing of the slurry components, the concrete should be placed as desired before the slurry becomes too stiff to be properly placed.
It is important that the proper ratios of coarse aggregate, fine aggregate, cement and water be used in preparing the concrete slurry. The concrete slurry must be sufficiently workable for proper placement in the construction application, yet the hardened concrete must possess the required durability and strength for the application. A mixture which does not have sufficient paste to fill the voids between the aggregate components will be difficult to place and will produce rough honey-combed surfaces and porous concrete. However, a mixture with excess paste will be smoother and easier to place, but it is subject to shrinkage and is more expensive. Therefore, the methods of providing concrete to a job site must maintain the proper proportions of each of the components of the concrete. There are generally three different known methods for providing concrete to a construction site. In the first method, pre-measured sacks of dry cement and aggregate are delivered to the job site, where the sack is opened and mixed with water to create the concrete slurry. This method has the advantage of allowing the slurry to be mixed shortly before placement, allowing substantial time for placement of the slurry before the concrete begins to stiffen. However, this method has the disadvantage of being costly and labor intensive. Individual sacks of dry concrete are more expensive than concrete purchased in bulk. In addition to the added expense for packaging and handling, the aggregate in sack concrete must have a very low moisture content to prevent the cement from prematurely hydrating within the sack. The sacks are heavy, difficult to handle, and must be individually opened and mixed. A 94 pound sack of dry concrete when mixed with approximately 6 gallons of water yields less than 5 cubic feet of concrete.
It is to be appreciated that because a common cement truck holds 9.5 cubic yards of concrete slurry (i.e., approximately 256 cubic feet), one would have to mix over 50 individual sacks of cement to equal the volume of slurry delivered by a single cement truck. By way of example, a 4 inch thick 1800 square foot concrete pad requires over 22 cubic yards of concrete, requiring three cement trucks to deliver the concrete slurry. This same job would require mixing and placing approximately 120 sacks of cement. Because of these limitations, the sack method is generally limited to very small jobs.
The second method of providing concrete to a construction site is perhaps the most commonly used. In this method, concrete slurry comprising aggregate, cement and water is placed into cement trucks at a cement plant, and the trucks thereafter deliver the slurry to the job site. There are several disadvantages of this method. The concrete slurry should be poured within 90 minutes from the time the cement and aggregate are mixed with water. Therefore, the distance of the job site from the cement plant can limit or prevent use of this method. If the truck is delayed by traffic or other reasons and the concrete slurry not placed within the required time window, the concrete slurry cannot be used and it becomes waste material. Not only is the concrete lost, but it must then also be transported to a proper disposal site.
Typically, it is desired that concrete be delivered to the construction site first thing in the morning. Accordingly, demand for concrete at the cement plant is high in the early morning. A cement plant might have a capacity of loading 15 to 20 trucks per hour. Depending upon the demand, there may be congestion at the cement plant, with a large number of cement trucks idling and waiting for concrete. If a particular construction project has a large demand for concrete, the number of trucks required to deliver concrete can be large, consuming large amounts of fuel, and emitting pollutants.
The third method of delivering concrete to a construction site is only practical for very large construction projects. This method is to set up a portable plant on the job site, with separate bulk storage for each of the concrete components. The components are thereafter weighed, blended and mixed on the job site as required for the construction. While this method has the advantage of providing concrete on an as-needed basis, it is prohibitively expensive except for large projects.